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It uses SpiderMonkey.

If possible, will it be very complicated to use V8? Or is it just impossible?

It's not impossible, but I don't know why you'd want to use V8.

Spidermonkey compares very well with V8: http://arewefastyet.com/

(beats it in Sunspider, is roughly equal in others)

And it maintains the lead in ECMAScript6 support: https://kangax.github.io/compat-table/es6/